This application relates in general to optical measurement systems and, in particular, an apodizing filter system for reducing spot size in optical measurements and for other applications.
Ellipsometers, spectrophotometers and similar optical instruments direct a beam of polarized or unpolarized radiation consisting of one or more wavelengths of electromagnetic radiation from the microwave, infrared, visible or ultraviolet regions of the spectrum at a sample which is to be measured. The reflected radiation, the transmitted radiation or both are collected and from the collected signal inferences are made about the material, or other, properties of the sample.
When the properties of the sample vary from place to place, it is usually necessary to illuminate (or alternatively, sense the signal from) only a small region of the sample to avoid the measurements of the region of interest being confused by the signal detected from the surrounding material of differing composition. An example of this is the measurement of the thickness of a thin film on a surface of a semiconductor wafer. For the measurement to be made on a wafer containing integrated circuits (a product wafer) rather than on a special test wafer, it is necessary to measure in the small area containing the film to be measured. In a modern integrated circuit, a feature that is to be measured may have dimensions which are only a few microns to a few tens of microns in size.
Focusing the illumination or detection system onto such a small area of a sample can be done using standard microscope technology when incident radiation consists only of visible light wavelength and is directed at normal or near normal incidence to the sample. It is well understood how to design a lens system for visible wavelengths to minimize chromatic and other aberrations to focus down to a spot of the order of a few microns. However, many optical instruments use angles of incidence very different from normal to enhance sensitivity. In particular, ellipsometers are typically aligned at an angle of incidence close to the Brewster's angle for the material of interest (about 75 degrees for silicon). In such cases, the high angle of incidence spreads the diffraction and aberration effects over a larger area. To a first approximation, the spot is spread over a distance 1/cos (incident angle) times the distance in the normal incidence case. Even when the incident light strikes the sample at near normal incidence, some applications require spot sizes that are about one micron or less in size. In these cases, as well as in the non-normal incidence case, aperture diffraction contributes significantly to the spot size and has to be minimized.
One common technique to reduce or alter the diffraction pattern of a small spot image is the pinhole spatial filter. In this technique, the light source is focused onto an aperture that is just slightly larger than the diffraction limited resolution of the focusing lens system. The light cone is allowed to expand to fill a second lens system that then forms the small spot image. The optical sub-system just described is commonly known as a pinhole spatial filter (see, for example, Optics, M. Klein and T. Furtak, chapter 7.3, J. Wiley & Sons, 1986).
The limitations of a pinhole spatial filter include the following:
(i) The light source must be focused onto the aperture to a spot that is small compared to the resolution of the first focusing lens system given the numerical aperture that is desired. In practice, this usually limits use of a pinhole spatial filter to laser light sources; PA1 (ii) The pinhole aperture must be aligned to the image with extremely tight tolerances, and the alignment must be maintained over the life of the optical apparatus. This could be a problem in commercial instruments that must survive shipping and temperature fluctuations; and PA1 (iii) The first lens system must have good imaging qualities. Chromatic aberration precludes the use of a refractive lens when a broad band of wavelengths, especially UV to near IR, is used. A reflective lens system will tend to partially polarize the beam which is not desirable for an ellipsometer illuminator.
These limitations make it difficult to use a pinhole spatial filter in most broad-band optical systems, especially those that measure polarization.